1. Field of the Invention
The present invention relates to a demodulator of a digital audio signal, and more particularly to a digital audio signal demodulator which demodulates a digital audio interface format signal, being continually inputted with asynchronous serial bits, to a non-returnto zero (NRZ) signal, corrects error per frame with respect to the NRZ signal, and provides the error-corrected NRZ signal, being synchronized with a digital-to-analog conversion control signal.
2. Description of the Related Art
Generally, one frame of a digital audio interface format signal is composed of a header signal of 4 bits for discriminating left and right channel signals, a phase-modulated subcode of 8 bits, audio data of 16 bits, and specific codes of 4 bits. The digital audio interface format signal of 20 bits except the header has been phase-modulated.
Specifically, one frame of the digital audio interface format signal consists of 46T (wherein 2T is a one-bit time) with a sampling frequency of 5.6448 MHZ, and a signal transition always occurs within the one-bit time (=2T) in the 28-bit region of the digital audio interface format signal except the header region. The digital audio interface format signal utilizes a Bi-Phase-Mark type modulation. Each bit is determined to be "1" if a signal transition exists during the one-bit time (=2T), while "0" if not.
The digital audio interface format signal is mainly used for signal transmission between digital audio appliances, and has the advantage that it suffers almost no signal loss in comparison with analog audio signal transimission. Meanwhile, the digital audio interface format signal, due to the characteristics of its signal format, is capable of being demodulated even if its phase is inverted by 180.degree., and thereby it can be adopted to an infrared rays digital data transmitting/receiving system.
FIG. 2 is a block diagram of a prior art digital audio signal transmitting/receiving system.
According to the signal transmitting system of FIG. 2, an audio source i.e. converted into serial digital data by multiplexing left and right audio channel signals by means of an analog-to-digital converter 100, and the serial digital data is phase-modulated and formatted by a phase modulator and interface format generator 150. The formatted signal is pulse-position-modulated by a pulse position modulator (PPM) 200 and then provided to an infrared-emitting diode driving section 250, so that the modulated signal is transmitted in the air by an infrared rays transmitting section 300 wherein infrared-emitting diodes are arrayed.
Meanwhile, according to the signal receiving system of FIG. 2, the pulse-position-modulated digital audio interface format signal transmitted in the air is detected by an infrared rays receiving section 350, and then amplified by an amplifier 400 to compensate for the attenuation of the received signal. The pulse-position-modulated digital audio interface format signal is reproduced and restored to its original format by an interface format reproducing section 450. The restored digital audio interface format signal, which is composed of 28 bits except a header signal, is then demodulated to an NRZ signal, and only the audio data therein is outputted to a digital-to-analog converter 600.
Consequently, according to the prior art system, the control signal required to control the digital-to-analog converter 600 must be synchronized with the demodulated audio data in order that the digital-to-analog converter 600 reproduces the original audio source using the audio data and the control signal synchronized with the audio data.